SIZE DISTRIBUTIONS OF RADIOACTIVE AEROSOLS 165 Total beta and individual radionuclide concentrationson the counting date and the flight date, respectively, for the four flights are plotted as a function of altitude in Figs. 2 to 5. Data are plotted as the midpoint of each altitude band sampled. The vertical profiles of both total concentration and concentration within each size range and the activity — size distribution are remarkably similar for all cases. Thus Several general observations can be made. Total Beta Radioactivity Let us first consider the sum of the total beta activity values found in each of the three size fractions at each given altitude interval. The vertical profiles are nearly identical for all flights with a peak con- centration in the 15- to 21-km region followed by a sharp decrease with increasing altitude. This decrease is more pronounced in the samples of 1962 than in those of 1963. The predominance of beta activity in the 15- to 21-km altitude band corresponds well with the peak in the number distribution of the natural sulfate aerosol.? In the 1963 collections the total beta-activity altitude and size distributions are quite similar to those for total gammaactivity (0.1 to 1.0 Mev) except for the persistence of gamma activity in particles with r> 0.15 at 21 to 30 km in August 1963. The need for sensitive low-level beta-counting techniques is demonstrated particularly by the Sept. 25, 1962, flight for which observed total beta-activity results for the largest particles varied over three orders of magnitude. Size data show that most of the total beta activity is on particles with r< 0.15 yu, Within this range the highest percentage is associated with particles of radius 0.02 to 0.15 y. In general, less than about 5% of the total beta activity for total sample was found on particles with r>0Q.15 yu. The only exceptions to this are the 27- to 30-km altitude band for the March 1962 collection (about 9%) and the September 1962 collection that showed the influence of fresh debris by varying from 5 to 40% with decreasing altitude band in this particle size range. With few exceptions all flights gave vertical distributions of total beta activity for each of the size fractions that were Similar to each other and to that for the total of the combined fractions. Relatively higher levels of total beta activity, however, do persist on particles with r< 0.02 / in the upper two sampling intervals for the 1963 collections. Particles with r < 0.02 4 show a peak in beta activity at 21 to 27 km in the March 1962 and August 1963 collections. It should be noted that, even if one were to correct the total beta concentrations observed for the 9- to 15-km interval by assuming that all of the activity had been sampled above the tropopause, the peak in the 15- to 21-km interval would remain in all cases except for the largest particles in the March 1962 collection.